1. Field of the Invention
The present invention relates generally to the field of skin ridge pattern imaging, and has particular applications in imaging of skin pressed against a platen surface, such as in fingerprint imaging.
2. Background Art
Internal reflection from a prism surface is typically used to see and photograph the unique pattern of ridges on a fingerprint, handprint, or other skin surface. Known imaging methods include dark-field viewing and bright-field viewing. In dark-field viewing, the image is seen by light scattered from finger ridges contacting a platen surface, and unimpeded total reflection gives a view of a black background surface. In bright-field viewing the surface appears bright by total reflection in all areas of a platen where it is not contacted by ridges. The latter view normally requires additional components such as a lens to collimate the illumination falling on the prism surface. FIG. 1 shows an apparatus 100 adapted for bright-field viewing. A prism 102 is contacted by finger 104 (or other body member having a print pattern of ridges and valleys) on platen surface 106. Platen surface 106 can be a surface of prism 102 itself or a surface of a transparent protective film or other material optically coupled to a surface of prism 102. An illumination source 108 provides light through prism 102. Illumination source 108 may include one or more light-emitting diodes and may be located as shown, or may be located behind back prism face 110. White paint 112 may be applied to back prism face 110.
An optical axis 114 extends from prism 102 through first lens 116, aperture stop 118, and imaging lens 120 to imaging device 122. Optical elements including lenses 116 and 120 may be more complex; these lenses may be lens groups rather than single lenses and additional optical elements may be provided along axis 114 to improve image quality. The finger-contacted surface 106 is preferably viewed telecentrically by imaging device 122 so that sensitivity to ridge contact is uniform over the entire viewed area. FIG. 1 merely illustrates one way that an optical system can be configured to achieve this result. The view is made through first lens 116 that directs the collected light from the prism through aperture stop 118 into imaging lens 120 that forms an image of the contacted prism surface onto any suitable imaging device 122. Imaging device 122 may, for example, include photographic film or a digital image sensor array, such as a CCD or CMOS component. Preferably imaging device 122 is tilted at a suitable angle to achieve a sharp image of the entire finger area, as by the Scheimflug condition.
While this design is capable of obtaining useful skin pattern images, it requires substantial illumination from LED or other light sources, which take up space in the device and require substantial electrical power. Therefore, there is a need for an improved optical illumination and imaging system and method that overcomes these deficiencies.
The present invention will now be described with reference to the accompanying drawings. In the drawings, some like reference numbers indicate identical or functionally similar elements. Additionally, the left-most digit(s) of most reference numbers identify the drawing in which the reference numbers first appear.